1. Field of the Invention
The present invention relates generally to an optical wavelength converter system for converting a fundamental wave into a second harmonic, or for converting fundamental waves into a wave having a frequency equal to the difference between or the sum of the frequencies of the fundamental waves, and more particularly to an optical wavelength converter system employing an optical wavelength converter device made of an organic nonlinear optical material.
2. Description of the Prior Art
Various attempts have heretofore been made for converting the wavelength of a laser beam into a second harmonic, i.e., shortening the wavelength of a laser beam, using a nonlinear optical material. One well known example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk crystal type converter device as disclosed, for example, in Introduction to Optical Electronics, pages 200-204, written by A. Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K. K.), pages 200-204. This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence cannot be employed even if it has high nonlinearity.
To solve the above problem, a fiber type optical wavelength converter device has been proposed. The optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of a nonlinear optical material and surrounded by a cladding. One example of such an optical fiber is shown in the Vol. 3, No. 2, pages 28-32, Bulletin of the Microoptics Research Group of a Gathering of the Applied Physics Society. Recently, many efforts are directed to the study of a fiber type optical wavelength converter device since it can easily gain phase matching between a guided mode in which a fundamental is guided through the core and a radiated mode in which a second harmonic is radiated into the cladding. Also known, as disclosed in U.S. Pat. No. 4,820,011, is a two-dimensional optical waveguide type optical wavelength converter device which includes a slab-shaped optical waveguide of nonlinear optical material sandwiched between two substrates that serve as a cladding layer. An optical wavelength converter device comprising a three-dimensional optical waveguide of nonlinear optical material embedded in a single substrate that serves as a cladding layer is also known. These optical waveguide type optical wavelength converter devices have the same features referred to above.
Various proposals have been made in recent years to use monocrystalline organic nonlinear optical materials in the fiber and optical waveguide type optical wavelength converter devices. Since organic nonlinear optical materials have much larger nonlinear optical constants than those of inorganic optical materials, they can achieve a high efficiency with which the wavelength of an applied wave can be converted. Examples of such organic nonlinear optical materials include MNA (2-methyl-4-nitroaniline), mNA (metanitroaniline), POM (3-methyl-4-nitropyridine-1-oxide), urea, NPP [N-(4-nitrophenyl)-(S)-prolinol], NPAN {2-[N-(4-nitrophenyl)-N-methylamino]acetonitrile], DAN (2-dimethylamino-5-nitroacetoanilide), MBA-NP [2-N(.alpha.-methylbenzylamino)-5-nitropyridine], (as disclosed in Japanese Unexamined Patent Publication No. 60(1985)-250334, Nonlinear Optical Properties of Organic and Polymeric Materials, ACS SYMPOSIUM SERIES 223, edited by David J. Williams and published by American Chemical Society in 1983, Organic Nonlinear Optical Materials supervised by Masao Kato and Hachiro Nakanishi and published by CMC in 1985, Nonlinear Optical Properties of Organic Molecules and Crystals edited by D. S. Chemla and J. Zyss and published by Academic Press Inc. in 1987, and The Quality and Performance of The Organic Non-Linear Optical Material(-)2-(.alpha.-Methylbenzylamino)-5-Nitropyridine (MBA-NP), Vol. 65, No. 8, page 229, written by R. T. Bailey et al. and published by Optics Communications), and 3,5-dimethyl-1-(4-nitrophenyl)pyrazole,3,5-dimetyl-1-(4-nitrophenyl)-1,2,4 -triazole, 2-ethyl-1-(4-nitrophenyl)imidazole, 1-(4-nitrophenyl)pyrrole, 2-dimethylaminol-5-nitroacetoanilide, 5-nitro-2-pyrrolidiacetoanilide, 3-methyl-4-nitropyridine-N-oxide, etc., (as disclosed in U.S. patent application Ser. No. 263,977, now U.S. Pat. No. 4,982,112). For example, the wavelength conversion efficiency of MNA is about 2000 times higher than that of LiNbO.sub.3 which is an inorganic nonlinear optical material. If an optical wavelength converter device is made of MNA, then it can generate a laser beam of a short wavelength in a blue region which is produced as a second harmonic of a fundamental wave that is applied as an infrared laser beam emitted from a general semiconductor laser, which is small in size and low in cost.
It has been recognized that the fiber or optical waveguide type optical wavelength converter device, with its optical fiber core or optical waveguide made of an organic nonlinear optical material, is disadvantageous in that the wavelength conversion efficiency and the incident coupling efficiency for a fundamental applied thereto become greatly reduced with time. More specifically, when the ends of the organic nonlinear optical material contact a surrounding atmosphere such as air, it sublimes from the ends, either shortening its monocrystalline section or getting modified to the extent that the monocrystalline structure is lost. The three-dimensional optical waveguide type optical wavelength converter device has its optical waveguide embedded in a surface of the substrate. Therefore, the above problem is likely to occur since the surface of the organic nonlinear material, as well as the ends thereof, contacts the surrounding atmosphere.